Isolation and Characterization of Endogenous Crude Oil Surface Active Species and their Implication in the Formulation of Surfactants for EOR

Summary In SP (Surfactant Polymer) and ASP (Alkali Surfactant Polymer) EOR processes, the surfactant role is to reduce the oil/water interfacial tension down to extreme values (10-3 mN/m and lower). The surfactant blend has to be adapted to the specific conditions of each reservoir, i.e. to the temperature, water in place salinity and crude oil nature, through rigorous phase behavior studies. If the effects of temperature and salinity on surfactants phase behavior are well established, the impact of crude oil, and particularly of its endogenous surface active species, is much less understood. Naphthenic acids and asphaltenes are the two crude oil components families that are usually described as having interfacial activity. Their implication in physical-chemical problems linked to surface activities, as emulsion and foam formation in separators, is well documented. However, their role in the presence of surfactants for EOR applications has been much less studied, even if ASP processes take advantage of the global contribution of naphthenic acids salts to the reduction of oil/water interfacial tension. Gaining insights on the interfacial activity of these endogenous surfactants in the presence of synthetic detergents could help select and eventually design the most efficient EOR surfactants. In this work, several techniques have been used to isolate the surface active species of a medium-density oil: naphthenic acids have been isolated by liquid-liquid extraction, asphaltenes have been precipitated with n-heptane and global interfacial materials have been extracted by emulsification and by using the wet-silica method. These natural surfactants have been characterized from a physical-chemical (tensiometry, phase behavior in model systems) and from an analytical (mass spectrometry) point of view. The phase behavior of EOR surfactants with the crude oil cleared from these components has also been studied, with and without alkali. All these experiments allowed confirming the surface activity of these crude oil extracts. Moreover, their contributions to the interfacial activity in the presence of EOR surfactants have been evaluated and compared, which gives some insights on the role of these species in surfactant formulation optimization.